This application claims priority benefit under 35 U.S.C. xc2xa7 119 from European Patent Application 00201801.2, filed on 22 May 2000.
1. Technical Field
The invention relates to a display device comprising a matrix of pixels at the area of crossings of row and column electrodes, each pixel comprising at least a current adjusting circuit based on a memory element, in series with a luminescent element.
Such electroluminescence-based display devices are increasingly based on (polymer) semiconducting organic materials. The display devices may either luminesce via segmented pixels (or fixed patterns) but also display by means of a matrix pattern is possible. The adjustment of the pixels via the memory element determines the intensity of the light to be emitted by the pixels. Said adjustment by means of a memory element, in which extra switching elements are used (so-called active drive) finds an increasingly wider application.
Suitable fields of application of the display devices are, for example, mobile telephones, organizers, etc.
2. Description
A display device of the type described in the opening paragraph is described in PCT WO 99/42983. In said document, the current through a LED is adjusted by means of two TFT transistors per pixel in a matrix of luminescent pixels; to this end, a charge is produced across a capacitor via one of the TFT transistors. This TFT transistor and the capacitor constitute a memory element. After the first TFT transistor has been turned off, the charge of the capacitor determines the current through the second TFT transistor and hence the current through the LED. At a subsequent selection, this is repeated.
In this drive mode, the charge across the capacitor is adjusted in such a way that the LED is switched between two modi, namely the xe2x80x9chigh power modexe2x80x9d and the xe2x80x9clow power modexe2x80x9d, in which the mutual time ratio between the two modi determines the grey value. To adjust this mutual ratio accurately, many extra electronics are required, inter alia, a processor and converters. Moreover, dependent on the grey value, switching between the two modi must be effected at high frequencies. This leads to an increased power consumption and hence faster ageing. Moreover, artefacts occur in moving images.
It is, inter alia, an object of the present invention to provide a display device of the type described in the opening paragraph in which the above-mentioned problems occur to a lesser extent. To this end, such a display device is characterized in that the device comprises means at the area of a pixel for adjusting a current through the luminescent element, as well as a switch between a plurality of luminescent elements and a connection point for an operating voltage.
By means of the switch (for example, a TFT transistor or a bipolar transistor), the luminescent elements are provided with a current corresponding to the desired luminance. During adjustment of a part of the drive circuit, the switch may be closed, if desired. However, it is opened during a part of a frame period. Parts of this drive circuit (for example, the combination of a capacitor and a transistor) determine the ultimate current through the luminescent elements. Since the luminescent elements can now convey current for a much shorter time, they are preferably driven in the so-called constant efficiency range. Here, the efficiency of the LED as a function of the diode voltage is practically constant. With a shorter time of conveying current through the LED (on-time), the current at a given luminance is usually so high that the LED is driven in this constant efficiency range.
In a first embodiment, the means for adjusting a current through the luminescent element comprise at least one switching element between a column electrode and a connection point of the memory element.
A preferred embodiment of a display device according to the invention is characterized in that the column electrode can be electrically coupled to a current source, and in that such a further circuit is arranged between the column electrode and the connection point of the memory element that the current adjusting circuit substantially does not conduct during adjustment of the value of the current through the luminescent element. This limits the dissipation.
The further circuit is preferably electrically detachable from the adjusting switch, while a transistor of this further circuit, together with a transistor in the memory element in the coupled state, constitutes a current mirror. Notably when all switches are made in one process (for example, TFTs in polysilicon technology) this results in uniform properties (and thus adjustments) of the switches throughout the display surface area.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.